It has been usual practice to give chemical treatment to the surface of a metallic material used for a container for packing food or drink, such as a steel sheet, a tin plate made by coating a steel sheet with tin, or aluminum, to form an oxide or hydroxide film on its surface to improve its corrosion resistance and coating adhesion, particularly its coating adhesion as required when it is processed. An oxide film is formed by forming an oxide directly on the surface of a metallic material, or by forming a hydroxide on the surface of a metallic material and causing it to react with oxygen in the air to form an oxide. There is also a hydroxide reacting only slowly with oxygen in the air. For simplicity of description, these oxides, hydrous oxides and hydroxides will hereinafter be referred to merely as oxides. A method relying on dipping a metal plate in a treating solution or a method relying on electrolysis in a treating solution is employed as a method of forming an oxide film. The method relying on dipping is a simple and convenient method of treatment, but is likely to be able to form only a film having too small a thickness to exhibit any satisfactory corrosion resistance or coating adhesion as intended. The method relying on electrolysis, which forms an oxide film instead of a film of metal plating, involves difficulty in achieving an adequate control of various conditions including the bath composition containing an oxidizing agent, its pH and the conditions of electrolysis and is not beneficial from a cost standpoint, either, as it requires a larger amount of electricity than metal plating.
Technique as shown below is, for example, disclosed as a technique for forming an oxide film on the surface of a metal plate. Patent Literature 1 discloses a method in which not a tin plate, but a DI tin can made by drawing and ironing a tin plate is brought into contact with a surface treating solution containing a water-soluble composition containing a phosphate ion, a condensed phosphate ion and a water-soluble polymer so that corrosion resistance and paint adhesion may be imparted to the can surface before coating or printing, but as it is a method of forming a film on the surface of a can body after its processing, and is not intended for improving the adhesion of a coating during its processing, but can form only a very thin film, it is not applicable as a method for chemical treatment of a flat sheet yet to be processed.
Patent Literature 2 discloses a method of forming a considerably thick film on a metallic material including a tin-plated steel sheet by its surface treatment with a metal surface treating agent for a precoated steel sheet containing a silane coupling agent and/or a hydrolysis condensate thereof, water-dispersible silica and a zirconium compound, but when this metal surface treating agent is applied to a tin-plated steel sheet used as a can material, no satisfactory corrosion resistance can be obtained if it is used without the addition of any water-dispersible silica as it makes a film too thick.
The following is the prior art literature information pertaining to the present application:    Patent Literature 1: Official Gazette JP-A-Hei-09-031403    Patent Literature 2: Official Gazette JP-A-2001-240979